Proper control of the immune system is critical to human health. A hypersensitive immune system can lead to allergies and autoimmune disease, whereas impairment of the immune system leaves an individual susceptible to infections and even cancers. Dendritic cells (DCs) play an important role in ensuring proper regulation of the immune system. In response to pathogens, DCs undergo rapid changes to limit the spread of infection, and initiate an adaptive immune response. Plasmacytoid DC (pDC) is a major subset of DCs and are one of the earliest defenses of the body against specific pathogens. Because of their importance to health and disease, as well as their therapeutic potential, it is important to understand how pDCs function. Here, I propose to investigate a relatively new and poorly understood area of pDC biology, microRNA (miRNA) regulation. miRNAs are an emerging class of small, non-coding RNAs that function in an important post-transcriptional regulatory network. miRNAs bind to mRNAs with near-complementarity and prevent their translation, thereby helping to control the molecular regulation of the cell. I have shown that miR-126 is highly and specifically expressed in pDCs and that miR-126 knockout mice have decreased pDC numbers as well as functional defects. I hypothesize that miRNA regulation also plays an important role in human pDC function. To test my hypothesis, I will first confirm that miR-126 is acting cell intrinsically in the mice models that I am using. I will also knockdown miR-126 in human dendritic cells by using miRNA decoy vectors, which are able to interfere with the ability of a specific miRNA to regulate its endogenous targets and thereby impair its function, and assaying pDC function. I will also investigate the mechanism by which miR-126 exerts its effects by identifying the cellular targets of miR-126. Finally, I will apply a novel assay that I have previously developed to examine the activity level of all conserved miRNAs to human dendritic cells. By employing these novel techniques to human dendritic cells, I will identify miRNAs with a role in human DC function, increasing our understanding of a powerful and previously unappreciated mechanisms of functional regulation in DCs. Our investigations into the miRNA-mediated regulation of human DC function will identify potential new causes of immune dysfunction, and supply new factors which may be targeted to either enhance or subdue an immune response.